This invention relates generally to high power lasers and, more particularly, to high power lasers in which many fiber lasers are combined in a single array. Combining the outputs of optical fiber lasers is emerging as a preferred technique for obtaining high powers from fiber lasers for both military and industrial uses. The fiber lasers are optically pumped to produce outputs that are combined and collimated into a single composite output beam of usefully high power. Such lasers producing output beams of several hundreds of watts power are already available. The present invention is more concerned with fiber laser arrays that can produce output powers measured in tens of kilowatts.
Fiber lasers in such arrays must be controlled in phase to produce a coherent composite output beam. There are various techniques for achieving beam coherency and thereby optimizing beam quality but the present invention is not concerned with these techniques. A critical feature of an array of fiber lasers is a lens array to which the fibers are coupled. Typically, each fiber is bonded to its own beam expander or end-cap in the array, which may have an integral or separate lens at the end of it. These individual end-caps take the form of cylindrical rods. Light entering one end of a rod or cylindrical end-cap from a fiber diverges as it passes along the rod and is collimated as it passes through one or more curved faces at the other end. The length and diameter of the cylinders are selected in accordance with the power emitted by a single fiber laser. Fiber lasers with higher output powers require the use of longer and thicker rod lenses, to allow the fiber output to diverge.
One factor that limits the total output power passing through a fiber lens array is the tendency of the array to overheat during prolonged periods of operation. Although the power dissipated in each element of the array is only a small fraction of the total power transmitted through the end-cap, over time even this low level of power dissipation heats the array to an unacceptable level, especially near the center of the array. Because the lens array must necessarily be as compact as possible, conventional cooling approaches do not address this problem.
Accordingly, there is a need for a fiber lens array structure that overcomes this problem.